Cells capable of converting sunlight into electric power, that is solar cells, have been receiving attention as energy sources to replace fossil fuels. Nowadays, some solar cells including crystalline silicon substrates and thin-film silicon solar cells are now being commercialized. However, the former solar cells have the problem of high production costs of silicon substrates. The latter thin-film solar cells have the problem of high production costs due to the need to use various types of gases for use in the production of semiconductors and complicated devices. Thus, in any type of solar cell, continuing efforts have been made to increase photoelectric conversion efficiency in order to reduce the cost per power output. However, the foregoing problems have not sufficiently been solved.
As a new type of solar cell, a wet-type solar cell using photoinduced electron transfer in a metal complex (for example, PTL 1 (Japanese Unexamined Patent Application Publication No. 01-220380)) and a wet-type solar cell using quantum dots (for example, PTL 2 (Japanese Unexamined Patent Application Publication No. 2008-287900)) are reported. In each of the wet-type solar cells, an electrode is provided on a surface of each of two glass substrates. The two glass substrates are arranged with these electrodes facing inward in such a manner that a photoelectric conversion layer is sandwiched between the electrodes. The photoelectric conversion layer contains a photoelectric conversion material on which a photosensitizing dye adsorbs to have an absorption spectrum in the visible light region and an electrolytic material. Such wet-type solar cells are also referred to as “dye-sensitized solar cells”.
The irradiation of the dye-sensitized solar cells with light generates electrons in the photoelectric conversion layer. The generated electrons move to one electrode through an external electric circuit. The electrons moved to the electrode are transported by ions in an electrolyte to opposite electrode and return to the photoelectric conversion layer. Electric energy can be taken from such a flow of electrons.
PTL 3 (Japanese Unexamined Patent Application Publication No. 2001-283941) discloses a solar cell in which a glass provided with a transparent conductive film is not used as a substrate on the light-receiving side. This solar cell includes at least a porous semiconductor layer, a conductive layer, a catalyst layer, and a counter electrode stacked in that order on a glass substrate. The solar cell does not include an expensive glass provided with transparent conductive film, thus enabling a reduction in cost. In addition, the absorption of light by a transparent conductive film can be blocked. This increases the quantity of light incident on a photoelectric conversion element, thereby increasing a current generated.